Recent developments in wide-bandgap power electronics have led to significant improvements in the power to size ratio. However, the passive magnetic components have shown less significant changes in recent years, and now represent a disproportionate amount of space and weight in the system. This has become a pressing issue for modern applications, e.g., small sizes and high switching frequencies are needed for notebook computers. See M. Koeda et al., Electr. Commun. Jpn. 96, 46 (2013). Furthermore, reduction in power consumption and enhancing overall efficiencies has become more imperative as the drive for a low-carbon economy continues. Rapid advancement in soft magnetic materials for the next generation of power electronics is therefore sorely needed. Currently, carbonyl iron and associated ferrites are used extensively as powder cores for inductor applications in high-power circuits. However, these are characterized by losses from remanent magnetization and eddy current formation, effects that are particularly evident at high switching frequencies. More highly desired magnetic properties include high saturation magnetization and permeability, low conductivity (to avoid eddy current losses), and low magnetic hysteresis. Ultimately, the goal is to combine high magnetic saturation, Msat, with a nearly flat permeability response from DC up to several MHz, performance superior in terms of permeability and loss behavior to that offered by soft ferrites. See C. Beatrice et al., J. Magn. Magn. Mater. 420, 317 (2016).
All of these design criteria can be met by an appropriately designed soft magnetic material, which avoids the common sources of loss. Superparamagnetic nanoparticles are a class of material that have seen intense research interest in fields including drug delivery, bimodal imaging, biosensing, and heavy metals recovery. See K. Mandel et al., ACS Appl. Mater. Interfaces 4, 5633 (2012); W. J. Dong et al., Adv. Mater. 23, 5392 (2011); W. L. Gu et al., Anal. Chem. 87, 1876 (2015); and L. J. Zhu et al., J. Controlled Release 169, 228 (2013). Superparamagnetism is a phenomenon that occurs in single domain particles, where the collective behavior of atomic spins leads to a giant vector spin that can randomly orient with sufficient thermal energy, leading to a net zero magnetization for the particle ensemble. Superparamagnets are defined by an absence of magnetic hysteresis, which makes them especially suitable for high frequency switching applications. The size of the particle required for superparamagnetism to emerge is also relatively small, which eliminates the contribution from eddy current loss, as the nanoparticles themselves are too small to support eddy currents. Therefore, superparamagnetic nanoparticles, in theory, should completely remove the two major sources of loss when compared to conventional core materials.
A strong candidate for effective application are iron nanoparticles. Iron is low-cost, being the fourth most abundant element, and is non-toxic. It possesses the highest room temperature Msat of any element (218 Am2/kg@293 K), while also possessing a very low magnetocrystalline anisotropy, meaning superparamagnetism can be observed at larger nanoparticle sizes. See B. Cullity, Introduction to Magnetic Materials, Addison-Wesley Pub. Co., Reading, Massachusetts (1972). This is important when optimizing the material's Msat as magnetization reduces with decreasing nanoparticle size. This is typically due to the formation of magnetically inert layers at the surface arising from spin-glass formation, or from surface electronic effects. See D. L. Huber, Small 1, 482 (2005).
In order to take advantage of superparamagnetic iron nanoparticles in core applications, it is necessary to separate the particles and prevent magnetic interaction. For example, magnetic dipole-dipole interactions can introduce hysteresis into a superparamagnetic ensemble. See M. Knobel et al., J. Nanosci. Nanotechnol. 8, 2836 (2008). Separation has previously been achieved by the formation of a polymer nanocomposite, in which the nanoparticles are suspended in a polymer matrix. See J. Pyun, Polymer Reviews 47, 231 (2007). Polymer nanocomposites have attracted significant research interest due to facile formation, light weight, and low cost of the matrix fraction. Furthermore, the plethora of different polymer and nanoparticle species available allows for the design of materials with tunable mechanical, magnetic, optical, and electrical properties. See H. Wakayama and H. Yonekura, Mater. Lett. 171, 268 (2016). This has led to a number of useful applications for nanocomposites including sensor applications, as conducting membranes for fuel cells, and as fire retardants. See J. Pyun, Polymer Reviews 47, 231 (2007). Polymeric nanocomposites can also be applied relatively easily to molds and also see promise in additive manufacturing. See A. C. de Leon et al., React. Funct. Polym. 103, 141 (2016). The real promise of nanocomposites however lies in their multi-functionality and the possibility of realizing unique combinations of properties unachievable with traditional, bulk materials. The inherent challenges in their formation include control over the distribution in size and dispersion of the nanostructured constituents, as well as tailoring and understanding the role of interfaces on the emerging bulk properties. Phase separation is also a well-established concern, which for a magnetic nanocomposite would eliminate the benefits of superparamagnetism due to the formation of ferromagnetic domains. See J. B. Hooper and K. S. Schweizer, Macromolecules 39, 5133 (2006). By their nature however, polymers have very large molecular weights, and are typically benign in terms of functionality. This means that in a nanocomposite the functional component becomes the minority fraction, which limits performance. Increasing this fraction to achieve high nanoparticle loadings, while still possessing control over the interparticle spacing and magnetic interactions would significantly increase the performance and applicability of nanocomposites.
Recently, a supramolecular building block approach has been suggested for the preparation of a new family of nanocomposites. These nanocomposites are comprised of nanoparticles cross-linked by covalently bound organic bridges, eliminating the need for a polymer matrix. See V. N. Mochalin et al., Acs Nano 5, 7494 (2011); and B. I. Dach et al., Macromolecules 43, 6549 (2010). The nanoparticles are separated by the surfactant molecules bound to their surfaces, which are covalently bound to neighboring nanoparticles through their corresponding surfactant molecules. In doing so, a “matrix-free” nanocomposite is formed. These nanocomposites are not prone to the nanoparticle aggregation effects that plague conventional nanocomposites, and provide exceptionally high strength and toughness. See V. N. Mochalin et al., Acs Nano 5, 7494 (2011).
A promising approach to forming matrix-free nanocomposites is by employing epoxy chemistries, as this is well-known to provide strong mechanical properties in a cross-linked environment. Epoxy resins are a class of thermosetting polymers that are ubiquitous as coatings, adhesives, and in structural repair and are recently seeing application in additive manufacturing applications. See B. G. Compton and J. A. Lewis, Adv. Mater. 26, 5930 (2014); and F. L. Jin et al., J. Ind. Eng. Chem. 29, 1 (2015). They have also been used on numerous occasions to form traditional nanocomposite materials. Carbonyl iron-epoxy magnetic cores have recently been used by Sugawa for large-current inductors mounted directly onto a chip. See Y. Sugawa et al., IEEE T. Magn. 49, 4172 (2013). They showed that good dispersion within the epoxy matrix leads to lower losses at high frequencies, due to less large magnetic agglomerates present in the system. Gu surface functionalized magnetite nanoparticles with conductive polyaniline to increase the epoxy-nanoparticle interaction and strengthen the nanocomposite mechanical properties. See H. Gu et al., ACS Appl. Mater. Interfaces 4, 5613 (2012). Incorporation of the functionalized nanoparticles led to better thermal stability as well as increased dispersion of magnetic fraction. Zhu formed magnetic epoxy nanocomposites with Fe@FeO core-shell nanoparticles. See J. H. Zhu et al., ACS Appl. Mater. Inter. 2, 2100 (2010). They used a commercially available epoxy system and formed nanocomposites with nanoparticle packing fractions of between 1 and 20 wt. %. They measured an Msat of 108 Am2/kg for the Fe@FeO nanoparticles themselves, which was reduced to 17 Am2/kg when incorporated into the epoxy network. Pour also showed improved mechanical properties when incorporating surface modified maghemite α-Fe2O3 nanoparticles into a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy matrix. See Z. S. Pour and M. Ghaemy, Prog. Org. Coat. 77, 1316 (2014). This was due to improved nanoparticle dispersion, and increased interfacial adhesion between the DGEBA and α-Fe2O3. However, maximum nanoparticle loading was only 11 wt %. While providing good examples of the usefulness an epoxy network in the formation of nanocomposites, these approaches mimicked the use of polymers in that the nanoparticles were embedded in an epoxy matrix.
In terms of the nanoparticle fraction, control over the size and shape is essential to produce an effective superparamagnetic nanocomposite. For example, a finite size distribution leads to a distribution in relaxation times, which can adversely affect performance in high frequency switching applications. See B. T. Naughton et al., J. Am. Ceram. Soc. 90, 3547 (2007). When considering shape, any deviation from an ideal sphere can introduce higher-order multipole terms in the magnetic dipole interaction energy, leading to deviations from the expected magnetic behavior. See N. Mikuszeit et al., J. Phys. Condens. Mat. 16, 9037 (2004). Controlling interparticle spacing is imperative; too close and interparticle interactions can lead to hysteresis and losses, too far and porosity can reduce the maximum achievable magnetic fraction; reducing the overall Msat of the nanocomposite. See B. T. Naughton et al., J. Am. Ceram. Soc. 90, 3547 (2007). Finally, the magnetic nanoparticles employed in the formation of the nanocomposite must be synthesized in sufficiently large quantities. This is especially important when considering application of the nanocomposite in inductor and transformer technologies, where the form-factor for testing can vary significantly.